In recent years, the development of a radio base station capable of being installed indoors or the like has been in progress. Since the coverage of a cell formed by this radio base station is extremely smaller than that of a radio base station installed outdoors (in the order of several meters to several tens of meters), the cell is generally called “femto cell”. In the 3GPP (3rd Generation Partnership Project), a radio base station that forms a femto cell is standardized as “HNB (Home Node B)”. Note that in the following explanation, a radio base station that forms a cell that is much larger than the femto cell (in general, called “macro cell”) may be called “MNB (Macro Node B)” and thereby differentiated from the HNB.
Further, the HNB is connected to an HNB-GW (HNB Gateway) through a public network such as the Internet, and thereby incorporated into a mobile communication network (CN: Core Network). The HNB-GW relays traffic between the CN and the HNB.
Furthermore, since the HNB can be installed more easily in comparison to the macro base station, the use of the HNB for delivering presence information (e.g., information specific to the place in which the HNB is installed) or the like has been studied as well as the use of the HNB in dead zones (introduction for supplementing the coverage of macro cells).
A procedure for detecting that a typical UE (User Equipment) camps on a femto cell (hereinafter “camping detection procedure”) and a procedure for detecting that the UE decamps from the femto cell (hereinafter “decamping detection procedure”), which are disclosed in e.g., NPL 1, will be sequentially described with reference to FIGS. 8 and 9.
[Camping Detection Procedure]
As shown in FIG. 8, a UE 40x transmits an RRC (Radio Resource Control) Connection Request, which is a type of RRC messages, to an HNB 30x in order to register its location on the femto cell (step S101). Note that in the following explanation, an RRC message may be expressed as “RRC:MMM (arbitrary message type)”.
At this time, if an IMSI (International Mobile Subscriber Identity) of the UE 40x is not notified to the HNB 30x, the UE 40x and the HNB 30x operate in cooperation to thereby perform a process for acquiring the IMSI (step S102). Specifically, the HNB 30x requests the UE 40x to notify the IMSI. In response to this request, the UE 40x notifies the IMSI to the HNB 30x. The IMSI is an identifier for uniquely identifying a subscriber all over the world and permanently assigned to a UE.
Then, the HNB 30x transmits a UE Register Request, which is a type of HNBAP (HNB Application Protocol) messages, to an HNB-GW 110x (step S103). The IMSI of the UE 40x is set in the UE Register Request message. Note that in the following explanation, an HNBAP message may be expressed as “HNBAP:MMM (arbitrary message type)”.
When the HNB-GW 110x receives the UE Register Request message, the HNB-GW 110x operates in cooperation with an AAA (Authentication, Authorization, and Accounting) server 120x to thereby determine whether or not the UE 40x is allowed to camp on the HNB 30x (step S104). Specifically, the HNB-GW 110x notifies the IMSI of the UE 40x to the AAA server 120x, thereby inquiring whether or not to allow the camping. The AAA server 120x holds in advance a database in which, for each of HNBs to be monitored, identification information (hereinafter “HNB-ID”) of each HNB is associated with IMSIs of UEs that are allowed to camp on each HNB. Therefore, the AAA server 120x refers to this database to determine whether or not to allow the UE 40x to camp on the HNB 30x. 
As a result, when it is determined that the UE 40x is allowed to camp on the HNB 30x, the HNB-GW 110x internally stores the IMSI of the UE 40x in association with the HNB-ID of the HNB 30x (step S105).
Then, the HNB-GW 110x transmits to the HNB 30x an HNBAP:UE Register Accept message that indicates the registration completion of the UE 40x and includes a Context-ID for the UE 40x (step S106). With these processes, the camping detection procedure has been completed. Therefore, the UE 40x can receive a desired radio communication service(s) within the femto cell.
[Decamping Detection Procedure]
As shown in FIG. 9, after a voice service, a packet service and the like are terminated, the connection which has been established for each service is disconnected under the initiative of the CN 20x (step S201). At this time, the HNB-GW 110x transmits to the HNB 30x a Disconnect, which is a type of RUA (RANAP User Application) messages, thereby releasing the connection between the HNB 30x and the own HNB-GW 110x (step S202). Note that in the following explanation, an RUA message may be expressed as “RUA:MMM (arbitrary message type)”.
When the HNB 30x receives the Disconnect message, the HNB 30x starts, on the own HNB 30x side, a timer (hereinafter referred to as “periodic location registration timer”) which is also used by the UE 40x for transmitting a Location Updating Request message at regular intervals, thereby monitoring whether or not the UE 40x has decamped from the own HNB 30x (whether or not the UE 40x has moved from the femto cell formed by the HNB 30x to another cell) (step S203).
On the other hand, when the UE 40x which is in an idle state detects that a femto cell under control of another HNB-GW (or macro cell) has a better quality than that of the femto cell formed by the HNB 30x, the UE 40x moves to the femto cell under control of the another HNB-GW (or macro cell), thereby decamping from the HNB 30x (step S204). As a result, as shown by the dotted line in FIG. 9, a camping detection procedure similar to the one shown at the above-described steps S101 to S106 is performed under the control of the another HNB-GW.
After that, when the HNB 30x detects the expiration of the periodic location registration timer, the HNB 30x determines that the UE 40x has moved to another cell (step S205). Then, the HNB 30x transmits an HNBAP:UE De-register message including the Context-ID for the UE 40x to the HNB-GW 110x, thereby requesting to deregister the UE 40x (step S206).
Upon receiving the UE De-register message, the HNB-GW 110x detects that the UE 40x has decamped from the HNB 30x (step S207). At this time, the HNB-GW 110x deletes the IMSI and the HNB-ID stored at the above-described step S105. With these processes, the decamping detection procedure has been completed.